A current practice by individual, institutional, industrial, and municipal consumers for the production of soft water is to use fixed-bed ion exchange resins, usually a sulfonated cation exchange resin such as a styrene-divinylbenzene copolymer. Hydraulic considerations currently limit resin particles to a size which gives maximum capactiy with an acceptable pressure drop at high flow rates. Most ion exchange resins used currently are generally sperical in shape and have diameters of 300 to 1000 microns (i.e., 20-50 mesh, U.S. Standard Screens).
However, the kinetics of 20-50 mesh resins impose limitations on column design that could be eliminated or at least significantly moderated by using a finer mesh resin. Fine mesh resins having diameters of only 15-20 microns (rather than the 300 to 1000 micron diameter resins now in use) have ion exchange rates on the order of 15 times faster than the conventional larger diameter resins and more efficient use of the ion exchange capacity. However, they have not been found acceptable for commercial use in the past because of hydraulic considerations. In fixed beds, fine mesh ion exchange resins cause excessive pressure drops, are prone to clogging and fouling, and are extremely difficult to backwash because they are easily carried out of the ion exchange column in the backwash cycle.
For many years the art has attempted to solve these problems so that advantage could be taken of the faster exchange rates achievable by using resins with increased surface area. For example, U.S. Pat. No. 2,460,516 to Luaces suggested that an ion exchange resin be deposited on the surface of a porous body to increase the surface area available during water softening. Voigtman, U.S. Pat. No. 2,798,850, disclosed coating felted or bat-type fibrous materials such as cellulosics, glass, or asbestos with various ion exchange resins to increase their exchange capacity.
Others have encapsulated magnetic particles in ion exchange resins. Examples of this are Weiss et al., U.S. Pat. Nos. 3,560,378, Turbeville, 3,657,119, and Weiss et al., 3,890,224. Weiss et al. 3,560,378 recognized the problems that fine ion exchange resins exhibited such as excessive pressure drop, quick fouling, and loss through entrainment. Their solution, however, was to use the encapsulated magnetic resins in an agitated mixer system during liquid treatment and then to magnetically coalesce the resin particles after treatment. Weiss et al. 3,560,378 did not purport to solve the problems associated with fine mesh resins when used in a fixedbed process. They did compare the reaction kinetics of gamma iron oxide particles encapsulated with trimethylol phenol N,N bis(3-amino propylmethylamine) having a particle size range of 250-500 microns with a standard size 350-1200 micron resin in fixed bed operation and found them to be substantially the same. However, no data on bed size, flow rates, or pressure drops was reported.
Svyadoshich et al. in "Wastewater Purification Using Superparamagnetic Dispersed Ion Exchanger in Constant Magnetic Field", 10 Soviet Inventions Illustrated 2 (#41 Nov. 1976), used a column surrounded by an electromagnetic coil which produced a magnetic field of 350 Oersted and a super-paramagnetic cation exchange resin (identified only as KU-2-8-f) 40-60 microns in diameter to obtain ion exchange rates eight times faster than conventional size resins.
In the field of water purification, attempts have been made to use high-gradient magnetic fields to separate and extract weakly paramagnetic submicron particles from fluid streams. DeLatour and Kolm, "High-Gradient Magnetic Separation: A Water Treatment Alternative", J. Am. Water Works Assoc. 325-327 (June 1976), discussed a number of suggestions for separation including possible use of a matrix of stainless steel wool in a column under the influence of a magnetic field to capture and hold magnetic particles from a fluid stream.
However, none of the above-mentioned prior art has satisfactorily solved the problems associated with fine mesh resins in fixed-bed columnar operation. Accordingly, the need still exists for increasing the efficiency of ion exchange processes which use fixed-bed columnar operation and yet will avoid the problems associated with fine mesh ion exchange resins when used in such columns.